The hypothesis of this application is that activation of PAR1 is a crRical step in prostate cancer tumorigenesis and inhibition of this step can lead to an effective treatment for prostate cancer. Effective treatment of prostate cancer will come from understanding the biology of this disease and applying this knowledge to the application of specific therapies. The development of new interventions for this disorder requires identification of specific targets to attack in men with prostate cancer. There are three major problems in attempting to treat prostate cancer. The first is the establishment of relevant models to test therapeutic agents in the pre-clinical setting before moving agents to the clinical setting. The second is the identification of therapeutic agents for intervention. The third problem is the optimal design of clinical trials that adequately test the agents identified in the pre-clinical setting. In the previous grant period, we have established in vitro and in vivo models of prostate cancer and used these models to identify potential targets and agents for intervention. This application focuses on PAR1 (protease activated receptor 1, the thrombin receptor) as a new target for therapeutic intervention in prostate tumorigenesis. In our preliminary data / progress report, we demonstrate PAR1 is overexpressed on prostate cancer cells as compared to normal prostate tissue by cDNA and tissue microarray. The Specific Aims of this proposal are as follows: Specific Aim 1: Characterize the functional role of the thrombin receptor (protease activated receptor 1, PAR1) in prostate cancer metastasis. We have identified that PAR1 is differentially expressed in prostate cancer cells and tissue compared to normal prostate. We will investigate the effects of blocking PAR1 in high-expressing VCaP cells and over expression of PAR1 in low-expressing DU145 cells in adhesion, motility, and invasion assays. Specific Aim 2: Test whether inhibitors of PAR1 prevent prostate cancer metastasis in preclinical models in vivo and establish benchmarks by which specific thrombin inhibitors will be chosen for future clinical trials in prostate cancer metastasis prevention and treatment. We will use in vivo model systems to test whether inhibitors of PAR1 decrease prostate cancer metastasis, growth and proliferation. Specific Aim 3: We will characterize the quantity and PAR1 phenotype of circulating prostate cancer cells in patients with hormone refractory prostate cancer to establish the basis for future clinical trials to examine PAR1 inhibitors for prostate cancer metastasis prevention and treatment. Levels of circulating cells will be measured in patients with metastatic prostate cancer, eventually prior to and after thrombin inhibitor administration if deemed to be appropriate. These proposed studies will characterize the role of PAR1 in prostate tumorigenesis and begin to define therapeutic strategies to treat prostate cancer by inhibition of PAR1 activation.